The field of the present invention relates to data capture devices, such as scanners and other optical code and object reading devices. In certain embodiments, optical readers and methods of reading are described herein which employ virtual scan lines.
A barcode label comprises a series of parallel dark bars of varying widths with intervening light spaces, also of varying widths. The information encoded in the barcode is represented by the specific sequence of bar and space widths, the precise nature of this representation depending on which particular barcode symbology is in use. Typical methods for reading barcodes comprise generation of an electronic signal wherein a signal voltage alternates between two preset voltage levels, one representing a dark bar and the other representing a light space. The temporal widths of these alternating pulses of high and low voltage levels correspond to the spatial widths of the bars and spaces. It is this temporal sequence of alternating voltage pulses of varying widths which is presented to an electronic decoding apparatus for decoding.
In one method of reading, referred to as a “scanning,” an illumination beam is moved (i.e., scanned) across the barcode while a photodetector monitors the reflected or backscattered light. For example, the photodetector may generate a high voltage when a large amount of light scattered from the barcode impinges on the detector, as from a light space, and likewise may produce a low voltage when a small amount of light scattered from the barcode impinges on the photodetector, as from a dark bar. The illumination source in spot scanners is a typically a laser, but may comprise a coherent light source (such as a laser or laser diode) or non-coherent light source (such as light emitting diode). A laser illumination source may offer advantages of higher intensity illumination which may allow barcodes to be read over a larger range of distances from the barcode scanner (large depth of field) and under a wider range of background illumination conditions.
Another method of reading is referred to as the “imaging” method whereby light reflecting off a surface is detected by an array (commonly a charge-coupled device or CCD) of optical detectors. The scan surface is typically illuminated by some uniform light source, such as a light-emitting diode (LED). Alternately, the illumination may be provided by ambient light such as disclosed in U.S. Pat. No. 5,814,803 hereby incorporated by reference. In such an imaging technique, as with a scanning laser, an electrical signal is generated having an amplitude determined by the intensity of the collected light. In either the scanning laser or CCD technique, the amplitude of the electrical signal has one level for dark bars and a second level for light spaces. As the label is scanned, positive-going and negative-going transitions in the electrical signal occur, signifying transitions between bars and spaces. Techniques are known for detecting edges of bars and spaces by detecting the transitions of the electrical signal. Techniques are also known for determining the widths of bars and spaces based on the relative location of the detected edges and decoding the information represented by the bar code.
An imaging system may comprise a linear (one-dimensional) array or may comprise a two-dimensional array of CCD elements to obtain an entire image of the optical code at one time. However, the drawback of these two-dimensional devices is that large amounts of memory are needed to store the image to be processed, and large amounts of computation are needed to extract the edge location data from the stored image. Further, complicated algorithms are necessary to determine the orientation and characteristics of the bar code label. One method of overcoming certain of these drawbacks is described in U.S. Pat. No. 6,142,376, hereby incorporated by reference, which discloses a method for reading encoded symbols such as bar code labels by making efficient use of area scanned input. Input data is provided by an area sensor such as a two dimensional CCD array or a video tube, or a single line sensor (such as a single line CCD or a single line laser scanner) combined with either movement of the item or an additional transverse scanning mechanism providing the second axis of the raster scan. Not all of the scanning data, but only a select portion of the data corresponding to virtual scan lines is stored and processed. A pattern of virtual scan lines is defined based on the dimensions of bar code labels to be scanned, the size of the scanning area, and the characteristics of the scanning sensor and optics. The virtual scan lines are mapped onto the raster pattern, allowing the pixels of the raster which fall on the virtual scan lines to be identified. As the raster scan proceeds, information regarding the intensity of the detected light is stored only for the pixels comprising the virtual scan lines. The amount of data stored is a fraction of the total data in the raster. After the pixel values have been stored for a virtual scan line, the edges of the bars and spaces crossed by the line is determined by a suitable method such as analog or digital processing.
Generally, there are typically two types of scanners: handheld or fixed. In a fixed scanner, there are two modes of operation: “sweep” mode and “presentation” mode. In sweep mode, bar coded items are moved through a scan volume in front of the scanner, the scanner typically taking advantage of the movement of the item to enhance potential read coverage. In presentation mode, the item may be held by the operator in front of the scanner, the scanner preferably producing a sufficiently omnidirectional scan pattern to read the bar code in any orientation.
In a handheld scanner, an operator holds the scanner and manually aims a scan pattern at a bar code. Handheld single line scanners, whether imaging or flying spot laser, require that an operator aim and orient the scanner relative to the bar code so that the scan line is substantially perpendicular to the bar code edges. Such operation requires some care on the part of the operator and reduces productivity. Furthermore, these devices are sensitive to label defects, as detection of bar and space edges is typically done along a single narrow scan line. To maximize the productivity of the operator and minimize stresses due to repetitive motions of the operator's body, and to minimize sensitivity to label defects, it is generally desirable to read bar codes which may be at any orientation relative to the scanning device.
In the fixed mode of operation, a relatively wide angular field of view is required so that a barcode label can be read from the largest possible fraction of the surface of the barcoded object. Since objects are often passed through the scan volume in random orientations, a multi-dimensional pattern is necessary to efficiently read the barcode. In addition, a high scan rate is desirable to allow successful reading of barcodes which are quickly passed through the scan volume.
A simpler scan pattern or a single scan line is often sufficient for portable operation, since the relatively small portable barcode scanner can be rotated to orient the scan line correctly across the barcode. A relatively small angular field of view and a relatively longer depth of field are desirable in this mode of operation. The longer depth of field may also allow the operator to read the barcode from a greater (or closer) distance. The reduced angular field of view reduces the likelihood of inadvertent scanning of other barcode labels, but in turn leads to tighter aiming requirements. To facilitate proper orientation of the scan line relative to the barcode and aiming of the scanner, the scan line may be made sufficiently intense to be seen by the operator. Alternatively, a portable barcode scanner may be provided with pointer illuminators to facilitate aiming of the barcode scanner.
There are also hybrid scanners designed to operate both in fixed and handheld modes of operation. U.S. Pat. No. 6,575,368, hereby incorporated by reference, discloses a dual mode laser scanner that is switchable between a handheld mode generating a single scan line and a fixed mode generating a multiple scan line omnidirectional scan pattern. In one embodiment, the '368 patent discloses generating both sets of scan patterns, but only decoding the single scan line during handheld mode of operation.
The present inventor has recognized there is still a need for versatile and effective data readers of different configurations.